Hydrogen-Bonding Structure at Zwitterionic Lipid/Water Interface

Interface structure of water/[3-palmitoyl-2-oleoyl-d-glycero-1-phosphatidylcholine] (POPC) lipid layer is investigated with molecular dynamics (MD) simulation by analyzing the recent heterodyne-detected vibrational sum-frequency generation (HD VSFG) spectroscopy. The MD simulation clearly reproduced the experimental HD VSFG spectrum of imaginary susceptibility (Im[-]), which exhibits two positive bands in the OH stretching vibrations of water. With the help of decomposition MD analysis, we found three kinds of interfacial water in relation to the HD VSFG spectrum. The low-frequency positive band is attributed to the water pointing toward the lipid side, whose orientation is influenced by negatively charged phosphate and positively charged choline of POPC. The high-frequency positive band is attributed to the water bonding with the carbonyl groups of the lipid. The gap between the two positive bands indicates the interfacial water pointing toward the bulk water phase in the vicinity of the choline groups.